Consideration of Aluminum for an Aircraft Engine Design

developed by Kenneth Kauffman

Background:

You have been hired by Thompson Consulting to offer your expertise in metallurgy for a jet engine design project. An experimental jet engine is being designed for a top secret project for the United States Military. The project is already over budget and the military sees only one possible solution: re-engineer the engine. The current proposal calls for ceramic components for part of the jet engine. However, ceramics are extremely expensive. The current design specifies a zirconia alloy. Thompson Consulting has determined that the tensile strength of the material must be greater than 21,000 psi. Further, the material used must be able to withstand repeat thermal cycling between room temperature and the normal operating conditions of the engine--625˚C. Sometimes, the material may be cooled down slowly (by contact with air at room temperature). However, in the case of a fire, it may need to be cooled rapidly (by a fire suppression system which sprays water at room temperature around the piece of concern). The piece must be able to operate at 625˚C, which means it cannot expand more than 2.5% from its size at room temperature. As a final concern, with the dust in the air and other possible debris which may get in the engine, the piece in question must have a hardness of at least 65 on the Knoop hardness scale to protect it from accidental damage, which could be disastrous at high altitudes.

You have been specifically asked to determine if the aluminum alloy 6061 will suffice for this job. In your report, consider each of the following points (among others which you feel are appropriate):

- are the hardness and tensile strength sufficient?

- is the cost of the aluminum alloy significantly less than the cost of the zirconia?

- are the thermal properties appropriate?

- is the fracture mode the same in the aluminum alloy as it is in the ceramic?

One group  is to conduct experiments to determine if the properties are acceptable for the sample before any thermal cycling occurs, one group is conduct experiments to determine if the properties are acceptable for the sample subjected to several rounds of heating and water quenching, and one group is to conduct experiments to determine if the properties are acceptable after heating and air cooling the aluminum alloy.

All groups should compare the final properties after cycling compare to the properties before cycling. If the aluminum alloy is not acceptable, propose another alloy that might be more acceptable. Provide a justification for your suggestion. All groups will need to turn in their results to the teaching assistant so they can be posted for all other groups.

The background section of your report should contain an explanation of each of the following topics in an appropriate level of detail:

- what is hardness and why is it a useful property to measure?

- what is tensile strength and why is it important in engineering applications?

- why are there different scales of hardness and how are comparisons made between them?

- why can heat treatments cause a change in physical properties?

- what are the current costs of the two raw materials under consideration?

- how are hardness and tensile strength related?

- what are the modes of fracture and when would each be desirable?

In your results and discussion section, determine how accurate the empirical relationship found in your textbook was for the data collected. Explain.

Procedure:

The procedures for the tensile and hardness tests follows. The TA will show you the proper way to operate the equipment.

Hardness of Materials

Procedure

Check the calibration of the hardness test machine by testing the hardness of a standard.  There are standards of known hardness available for both the "B" scale and the "C" scale

Form into groups.  Obtain a sample.  Lay out a grid on the sample.  Take a hardness reading at each grid point.  The grid should be suitable in size to allow a minimum of 50 readings on the sample. 

Construct a histogram of the data.  Plot the running mean.

The answers to the following questions may be of some help in completing the background section of the report.

What is hardness?

What is the principle of hardness measurement?

Name and describe the different methods of hardness measurement.

What is the relationship between hardness and tensile properties of metals?

Tensile Strength of Metals

Procedure

Determine the tensile properties of the given samples by the following procedure.

1.   Measure the cross-sectional width and thickness of the specimen and mark the gage length of 2 in.  Calculate the cross-sectional area.

2.   Check the tensile testing machine for appropriate loading, etc.

3.   Load the specimen. 

4.   Operate the tensometer according to the TA instructions.

5.   After the fracture of the specimen, measure the extension in the gage length.

6.   Calculate stress, and strain from the observed values of area, applied load, and data recordings.

7.   Plot the Stress-Strain curve and determine from the graph:

Ultimate Tensile Stress (UTS)

0.2 % Proof Stress  ( 0.2 % Offset Yield Stress )

Fracture stress

Young's Modulus

8.   Calculate the following:

% Elongation

% Reduction in area

9.   Sketch and describe each fracture pattern.

10. Take hardness readings along the length of the specimen.  Determine if the hardness varies significantly.

Other things to include in the background section of the report:

1. How do you use the Modulus of Elasticity and Yield Strength in engineering design applications?

2. What important information can be obtained from the % Elongation at fracture data for a metal or alloy?

3. Find specifications for the alloy designated.  Find UNS designations for the alloy where possible.  If not, find AISI-SAE specifications.  Be able to state what the specifications mean.

4. Find standards to compare with your results (UTS, Youngs modulus, etc).

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